1. Field of the Invention
The invention relates to the protection of articles made of alloy steel or a nickel-based superalloy against oxidation and frictional wear at temperatures up to 600.degree. C.
2. Summary of the Prior Art
Assemblies of components in turbo-machines frequently operate without conventional lubrication by oil circulation or greasing. Such assemblies, for example the mountings of compressor blades in the disc sockets, the main shaft splines, the passages for the disc tie-rods, the suspension axles, the joints, or even the shock absorbers of the high pressure (HP) compressor disc, are all places where there is local contact between parts which often entail slight movements and wear due to microclearances (rubbing). Furthermore, there are a number of cases of wear brought on by major relative movement. These phenomena can give rise to fatigue cracks and lead to premature breakage of parts.
Furthermore, one particular and just as troublesome kind of wear is observed in the compressor flow paths, i.e. wear by erosion.
All the types of wear and tear mentioned bring about deterioration of the surfaces and/or changes in the dimensions of the blade assemblies at the tips, at the leading edges, and at the trailing edges, the practical consequences being a reduction in the effective life of the blades and substantial losses in HP compressor performance. This sensitivity is bound up with the materials used for the blades, especially the moving parts.
For instance, studies carried out on HP compressors in operation have revealed that after 3000 flight cycles of 3 hours and 30 minutes, considerable losses of efficiency amounting to as much as 14% flow loss were due to the factors mentioned above, particularly blade erosion and blade tip wear.
Remedies to the problems of dry wear due to microclearances may be found by employing solid materials such as steels with a high carbon content, for example Z100 CD 17 (AFNOR standard--steel containing 1% C, 17% Cr and 0.5% Mo) with a high hardness level when cold, or forged cobalt-based alloys, cast or obtained by fusion. Coatings applied by plasma, such as tungsten carbides containing 17% cobalt, chromium carbides, and cobalt-based alloys can be used, but are applicable only in an homogeneous pairing in order to obtain sufficiently low friction coefficients.
The recourse to electrolytic coatings containing cobalt and about 30% chromium carbide indicated in French specification No. A-2 412 626 makes it possible to improve dry friction behaviour within a temperature range of between 300.degree. and 750.degree. C.
However, the processes mentioned above do not make it possible to resolve simultaneously the problems of wear due to microclearances and erosion.
The production of coatings using electrolysis is desirable in many cases, for example:
when the geometry of the parts to be treated is complex, plasma application of coatings may be difficult if not impossible (for instance in the case of small diameter bores);
whenever it is desired to apply thin, fairly homogeneous deposits;
when replacing certain sprayed coatings which, applied to parts which suffer from friction, might lead to deformation due to heating caused by the spraying process; and,
according to the type of parts involved, electrolytic coatings may be cheaper than plasma coatings and therefore offer a better efficiency/cost compromise.
The invention seeks to provide an electrolytic process which permits the codeposition of nickel and cobalt with ceramic particles to yield a hard composite material with a good level of resistance to wear and erosion.
U.S. patent specification No. 3 152 971 mentions the possibility of forming a codeposit of electrolytic nickel with powders of the ceramic type in order to produce an anti-reflective satinised finish coating, but this is an entirely different purpose from that with which the present invention is concerned. Furthermore, this document does not disclose the use of any nickel-cobalt-ceramic combination, nor does it show any means of optimising an homogeneous ceramic particle concentration in the coating or surfacing formed.
The difficulty with obtaining a nickel-cobalt codeposit is that for two or more species to be discharged simultaneously onto the cathode during electrolysis, the species must be in ionic forms such that they have closely aligned discharge potentials during deposition, which is not the case with nickel and cobalt. Obtaining a Ni-Co-ceramic electrolytic alloy thus requires:
reconciling the equilibrium voltages of the metals present in solution,
increasing the excess voltage of the most positive metal, and
reducing the excess voltage of the most negative metal.
One of the objects of the invention, therefore, is to find a compromise between the electrolysis conditions in order to bring about an effective electrolytic deposition of nickel and cobalt and to optimise the Ni/Co ratio during the process of electrolysis in order to obtain satisfactory dispersion of the ceramic particles in the final coating and also adequate homogeneity within the coating.